The present invention relates to a method of forming a solid-state image pick-up device, and more particularly to a method of forming micro lenses of a solid-state image pick-up device.
A two-dimensional charge coupled device is one of the typical solid-state image pick-up devices. The two-dimensional charge coupled device has an image pick-up region on which a plurality of photodiodes are aligned in matrix, wherein each of the photodiodes converts an optical signal into an electrical signal. A micro lens is provided over each of the photodiodes to improve sensitivity of the photodiodes. The micro lens is hemispherical-shaped. A light or a photon is transmitted through the micro lens and injected into the photodiode, whereby the light is converted into a charge which corresponds to the amount of light or photon. The generated charge is then accumulated into the photodiode.
In Japanese patent publication No. 60-59752, there is disclosed a conventional method of forming micro lenses of a solid-state image pickup device. FIGS. 1A through 1D are fragmentary cross sectional elevation views illustrative of solid-state image pick-up devices involved in a conventional method of forming an alignment of micro lenses over photodiodes of the solid-state image pick-up device,
With reference to FIGS. 1A and 1D, a p-type well region 2 is formed over an n-type semiconductor substrate 1. N-type photo receiving regions 3 are selectively formed in an upper region of the p-type well region 2. N-type charge coupled device channel regions 4 are also selectively formed in the upper region of the p-type well region 2, so that the n-type charge coupled device channel regions 4 are separated from the n-type photo receiving regions 3. P+-type device isolation regions 5 are also selectively formed in the upper region of the p-type well region 2, so that each of the p+-type device isolation regions 5 is positioned in contact with the n-type charge coupled device channel region 4 and the n-type photo receiving region 3 and also positioned between the n-type charge coupled device channel region 4 and the n-type photo receiving region 3, whereby the n-type charge coupled device channel region 4 is isolated by the p+-type device isolation region 5 from the n-type photo receiving region 3. A gate oxide film 6 is formed over the upper region of the p-type well region 2, the n-type photo receiving regions 3, the n-type charge coupled device channel regions 4 and the p+-type device isolation regions 5. Polysilicon gate electrodes 7 are selectively formed on the gate oxide film 6, wherein each of the polysilicon gate electrodes 7 extends to cover the n-type charge coupled device channel region 4 and a closer half region of the p+-type device isolation regions 5 to the n-type charge coupled device channel region 4 as well as cover the upper region of the p-type well region between the n-type photo receiving region 3 and the n-type charge coupled device channel region 4. The polysilicon gate electrodes 7 do not extend to cover the n-type photo receiving region 3 and the closer half region of the p+-type device isolation region 5 to the n-type photo receiving region 3. Shielding layers 8 are formed which cover the polysilicon gate electrodes 7 and the gate oxide film 6 around the polysilicon gate electrodes 7 except over center regions of the n-type photo receiving regions 3, so as to allow lights to be injected or incident into the center regions of the n-type photo receiving regions 3. The above structure from the substrate 1 to the shielding layers 8 will hereinafter be referred to as a base region 110. A planarized insulation layer 111 is entirely formed over the base region 110. The planarized insulation layer 111 comprises either a silicon oxide layer or a transparent resin layer. The planarized insulation layer 111 is transparent to the light. Each gap between adjacent two of the shielding layers 8 is filled with the planarized insulation layer 111. A plurality of micro lenses 112 are formed on the planarized surface of the planarized insulation layer 111, so that the micro lenses 112 are positioned over the n-type photo receiving regions 3 and also over the gaps between the shielding layers 8. The light is transmitted through the micro lens 112 and the planarized insulation layer 111 in the gap between the shielding layers 8 and incident into the center region of the n-type photo receiving region 3.
The above solid-state image pickup device with the micro lenses may be formed as follows.
With reference to FIG. 1A, a p-type well region 2 is formed over an n-type semiconductor substrate 1. N-type photo receiving regions 3 are selectively formed in an upper region of the p-type well region 2. N-type charge coupled device channel regions 4 are also selectively formed in the upper region of the p-type well region 2, so that the n-type charge coupled device channel regions 4 are separated from the n-type photo receiving regions 3. P+-type device isolation regions 5 are also selectively formed in the upper region of the p-type well region 2, so that each of the p+-type device isolation regions 5 is positioned in contact with the n-type charge coupled device channel region 4 and the n-type photo receiving region 3 and also positioned between the n-type charge coupled device channel region 4 and the n-type photo receiving region 3, whereby the n-type charge coupled device channel region 4 is isolated by the p+-type device isolation region 5 from the n-type photo receiving region 3. A gate oxide film 6 is formed over the upper region of the p-type well region 2, the n-type photo receiving regions 3, the n-type charge coupled device channel regions 4 and the p+-type device isolation regions 5. Polysilicon gate electrodes 7 are selectively formed on the gate oxide film 6, wherein each of the polysilicon gate electrodes 7 extends to cover the n-type charge coupled device channel region 4 and a closer half region of the p+-type device isolation regions 5 to the n-type charge coupled device channel region 4 as well as cover the upper region of the p-type well region between the n-type photo receiving region 3 and the n-type charge coupled device channel region 4. The polysilicon gate electrodes 7 do not extend to cover the n-type photo receiving region 3 and the closer half region of the p+-type device isolation region 5 to the n-type photo receiving region 3. Shielding layers 8 are formed which cover the polysilicon gate electrodes 7 and the gate oxide film 6 around the polysilicon gate electrodes 7 except over center regions of the n-type photo receiving regions 3, so as to allow lights to be injected or incident into the center regions of the n-type photo receiving regions 3. The above structure from the substrate 1 to the shielding layers 8 will hereinafter be referred to as a base region 110.
With reference to FIG. 1B, a planarized insulation layer 111 is entirely formed over the base region 110. The planarized insulation layer 111 comprises either a silicon oxide layer or a transparent resin layer. The planarized insulation layer 111 is transparent to the light, Each gap between adjacent two of the shielding layers 8 is filled with the planarized insulation layer 111.
With reference to FIG. 1C, a transparent thermo-setting resin material as a micro lens material is applied entirely on the planarized surface of the planarized insulation layer 111. The transparent thermo-setting resin material is patterned by a photo-lithography technique to form micro lens patterns 112a which are positioned over the n-type photo receiving regions 3 and also over the gaps between the shielding layers 8.
With reference to FIG. 1D, a heat treatment is carried out to cause thermal re-flow of the micro lens patterns 112a to form micro lenses 112 on the planarized surface of the planarized insulation layer 111, so that the micro lenses 112 are positioned over the n-type photo receiving regions 3 and also over the gaps between the shielding layers 8. The light is transmitted through the micro lens 112 and the planarized insulation layer 111 in the gap between the shielding layers 8 and incident into the center region of the n-type photo receiving region 3.
As described above, the micro lenses 112 are formed by softening the micro lens patterns 112a made of the transparent thermo-setting resin material by the heat treatment. The shape of the micro lenses 112 depends upon a heat history of the micro lens material, a surface state of the planarized insulation layer 111, and a size and a thickness of the micro lens patterns 112a. Variations in temperature of the thermal re-flow process, and in size and thickness of the micro lens patterns 112a cause variation in shape of the micro lenses 112. The micro lenses 112 are made of the transparent thermo-setting resin material. This means that the micro lenses 112 are thermally unstable. It is, therefore, difficult to realize a high re-productivity of the highly accurate shaped micro lenses 112.
In Japanese laid-open patent publication No. 5-320900, it is disclosed that in order to solve the above problem, trench cavities or ridges are formed to suppress the micro lenses from spread. FIGS. 2A through 2D are fragmentary cross sectional elevation views illustrative of solid-state image pick-up devices involved in a second conventional method of forming an alignment of micro lenses over photodiodes of the solid-state image pick-up device.
With reference to FIG. 2A, a p-type well region 2 is formed over an n-type semiconductor substrate 1. N-type photo receiving regions 3 are selectively formed in an upper region of the p-type well region 2. N-type charge coupled device channel regions 4 are also selectively formed in the upper region of the p-type well region 2, so that the n-type charge coupled device channel regions 4 are separated from the n-type photo receiving regions 3. P+-type device isolation regions 5 are also selectively formed in the upper region of the p-type well region 2, so that each of the p+-type device isolation regions 5 is positioned in contact with the n-type charge coupled device channel region 4 and the n-type photo receiving region 3 and also positioned between the n-type charge coupled device channel region 4 and the n-type photo receiving region 3, whereby the n-type charge coupled device channel region 4 is isolated by the p+-type device isolation region 5 from the n-type photo receiving region 3. A gate oxide film 6 is formed over the upper region of the p-type well region 2, the n-type photo receiving regions 3, the n-type charge coupled device channel regions 4 and the p+-type device isolation regions 5. Polysilicon gate electrodes 7 are selectively formed on the gate oxide film 6, wherein each of the polysilicon gate electrodes 7 extends to cover the n-type charge coupled device channel region 4 and a closer half region of the p+-type device isolation regions 5 to the n-type charge coupled device channel region 4 as well as cover the upper region of the p-type well region between the n-type photo receiving region 3 and the n-type charge coupled device channel region 4. The polysilicon gate electrodes 7 do not extend to cover the n-type photo receiving region 3 and the closer half region of the p+-type device isolation region 5 to the n-type photo receiving region 3. Shielding layers 8 are formed which cover the polysilicon gate electrodes 7 and the gate oxide film 6 around the polysilicon gate electrodes 7 except over center regions of the n-type photo receiving regions 3, so as to allow lights to be injected or incident into the center regions of the n-type photo receiving regions 3. The above structure from the substrate 1 to the shielding layers 8 will hereinafter be referred to as a base region 110. A planarized insulation layer 211 is entirely formed over the base region 110. The planarized insulation layer 211 comprises either a silicon oxide layer or a transparent resin layer. The planarized insulation layer 211 is transparent to the light. Each gap between adjacent two of the shielding layers 8 is filled with the planarized insulation layer 211, A photo-resist 13a is applied entirely over the planarized insulation layer 211. The photo-resist 13a is patterned by a photlithography to form a photo-resist pattern 13a having openings which are positioned over the n-type charge coupled device channel region 4.
With reference to FIG. 2B, the photo-resist pattern 13a is used to carry out a dry etching to the planarized insulation layer 211, so that the planarized insulation layer 211 is partially etched on boundary regions 211b to form trench cavities 221a on the boundary regions 211b of the planarized insulation layer 211. The used photo-resist pattern 13a is then removed.
With reference to FIG. 2C, a transparent thermo-setting resin material as a micro lens material is applied entirely on the planarized surface of the planarized insulation layer 211. The transparent thermo-setting resin material is patterned by a photo-lithography technique to form micro lens patterns 212a which are positioned over the top surfaces of the planarized insulation layer 211 except on adjacent regions to the trench cavities 221a and also expect over the trench cavities 221a. 
With reference to FIG. 2D, a heat treatment is carried out to cause thermal re-flow of the micro lens patterns 212a to form micro lenses 212 on the top surfaces of the ridged portions of the planarized insulation layer 211. The re-flow of the micro lens patterns 212a is limited at the edge of the ridged portions of the planarized insulation layer 211. As a result, the micro lenses 212 are positioned over the n-type photo receiving regions 3 and also over the gaps between the shielding layers 8. The light is transmitted through the micro lens 212 and the planarized insulation layer 211 in the gap between the shielding layers 8 and incident into the center region of the n-type photo receiving region 3.
As described above, the edges of the ridged portion of the planarized insulation layer 211 provide a limitation to the spread of the re-flow of the micro lens patterns 212a into the top portions of the ridged portions of the planarized insulation layer 211. Namely, the micro lenses 212 are formed by softening the micro lens patterns 212a made of the transparent thermo-setting resin material by the heat treatment. Variation in shape of the micro lenses 212 is suppressed by the edges of the ridged portion of the planarized insulation layer 211.
The above second conventional method of forming the micro lens has the following problems. The isolation trench cavities 221a are provided to isolate the individual ridged regions on which the micro lenses 212 are formed, so that the edges of the isolation trench cavities 221a suppress the spread of the re-flow of the micro lens patterns 212a, whereby the micro lens 212 is limited on the ridge defined by the isolation trench cavities 221a. This means that a width xe2x80x9cXxe2x80x9d of the micro lens 212 is accurately defined by the width of the ridge. The width and the size of the micro lens depend upon the material of the micro lens and the temperature of the re-flow process, for which reason even the width of the micro lens is accurately controlled or decided by the width of the ridge, it is difficult to accurately control or decide the height or thickness xe2x80x9cYxe2x80x9d of the micro lens. Further, if it is necessary to form size-different micro lenses over the base structure 110, then it is, however, difficult to realize the control in size differently of the micro lenses. In order to improve the sensitivity, it is effective to enlarge the size of the micro lenses and narrow the distance between the adjacent two of the micro lenses. It is, however, different to further narrow the isolation trench cavities isolating the ridges on which the micro lenses are formed. This means it difficult to further narrow the distance between the adjacent two of the micro lenses.
FIGS. 3A through 3D are fragmentary cross sectional elevation views illustrative of solid-state image pick-up devices involved in a third conventional method of forming an alignment of micro lenses over photodiodes of the solid-state image pick-up device.
With reference to FIG. 3A, a p-type well region 2 is formed over an n-type semiconductor substrate 1. N-type photo receiving regions 3 are selectively formed in an upper region of the p-type well region 2. N-type charge coupled device channel regions 4 are also selectively formed in the upper region of the p-type well region 2, so that the n-type charge coupled device channel regions 4 are separated from the n-type photo receiving regions 3. P+-type device isolation regions 5 are also selectively formed in the upper region of the p-type well region 2, so that each of the p+-type device isolation regions 5 is positioned in contact with the n-type charge coupled device channel region 4 and the n-type photo receiving region 3 and also positioned between the n-type charge coupled device channel region 4 and the n-type photo receiving region 3, whereby the n-type charge coupled device channel region 4 is isolated by the p+-type device isolation region 5 from the n-type photo receiving region 3. A gate oxide film 6 is formed over the upper region of the p-type well region 2, the n-type photo receiving regions 3, the n-type charge coupled device channel regions 4 and the p+-type device isolation regions 5. Polysilicon gate electrodes 7 are selectively formed on the gate oxide film 6, wherein each of the polysilicon gate electrodes 7 extends to cover the n-type charge coupled device channel region 4 and a closer half region of the p+-type device isolation regions 5 to the n-type charge coupled device channel region 4 as well as cover the upper region of the p-type well region between the n-type photo receiving region 3 and the n-type charge coupled device channel region 4. The polysilicon gate electrodes 7 do not extend to cover the n-type photo receiving region 3 and the closer half region of the p+-type device isolation region 5 to the n-type photo receiving region 3. Shielding layers 8 are formed which cover the polysilicon gate electrodes 7 and the gate oxide film 6 around the polysilicon gate electrodes 7 except over center regions of the n-type photo receiving regions 3, so as to allow lights to be injected or incident into the center regions of the n-type photo receiving regions 3. The above structure from the substrate 1 to the shielding layers 8 will hereinafter be referred to as a base region 110. A planarized insulation layer 211 is entirely formed over the base region 110. The planarized insulation layer 211 comprises either a silicon oxide layer or a transparent resin layer. The planarized insulation layer 211 is transparent to the light. Each gap between adjacent two of the shielding layers 8 is filled with the planarized insulation layer 211. A photo-resist 13a is applied entirely over the planarized insulation layer 211. The photo-resist 13a is patterned by a photo-lithography to form photo-resist patterns 13a which are positioned over the n-type charge coupled device channel region 4.
With reference to FIG. 3B, the photo-resist patterns 13a are used to carry out a dry etching to the planarized insulation layer 211, so that the planarized insulation layer 211 is partially etched on other regions than boundary regions 311b to form wide cavities defined between the adjacent two of the ridged portions positioned on the boundary regions 311b of the planarized insulation layer 311. The used photo-resist pattern 13a is then removed.
With reference to FIG. 3C, a transparent thermo-setting resin material as a micro lens material is applied entirely on the planarized insulation layer 311. The transparent thermo-setting resin material is patterned by a photo-lithography technique to form micro lens patterns 312a which are positioned in the wide cavities defined between the ridged portions 311a of the planarized insulation layer 311.
With reference to FIG. 3D, a heat treatment is carried out to cause thermal re-flow of the micro lens patterns 312a to form micro lenses 312 on the top surfaces of the ridged portions of the planarized insulation layer 311. The re-flow of the micro lens patterns 312a is limited at the edge of the wide cavities of the ridged portions of the planarized insulation layer 311. As a result, the micro lenses 312 are positioned over the n-type photo receiving regions 3 and also over the gaps between the shielding layers 8. The light is transmitted through the micro lens 312 and the planarized insulation layer 311 in the gap between the shielding layers 8 and incident into the center region of the n-type photo receiving region 3.
As described above, the edges of the wide cavities of the ridged portions of the planarized insulation layer 311 provide a limitation to the spread of the re-flow of the micro lens patterns 312a into the wide cavities defined between the ridged portions of the planarized insulation layer 311. Namely, the micro lenses 312 are formed by softening the micro lens patterns 312a made of the transparent thermo-setting resin material by the heat treatment. Variation in shape of the micro lenses 312 is suppressed by the edges of the wide cavities of the ridged portions of the planarized insulation layer 311.
The above third conventional method of forming the micro lens has the following problems. The ridges 311a are provided to define the individual wide cavities on which the micro lenses 312 are formed, so that the edges of the ridges 311a suppress the spread of the re-flow of the micro lens patterns 312a, whereby the micro lens 312 is limited within the wide trench cavities by the ridges 311a. This means that a width xe2x80x9cXxe2x80x9d of the micro lens 312 is accurately defined by the width of the wide trench cavity by the ridges 311a. The width and the size of the micro lens depend upon the material of the micro lens and the temperature of the re-flow process, for which reason even the width of the micro lens is accurately controlled or decided by the width of the wide trench cavity, it is difficult to accurately control or decide the height or thickness xe2x80x9cYxe2x80x9d of the micro lens. Further, if it is necessary to form size-different micro lenses over the base structure 110, then it is, however, difficult to realize the control in size differently of the micro lenses. In order to improve the sensitivity, it is effective to enlarge the size of the micro lenses and narrow the distance between the adjacent two of the micro lenses. It is, however, different to further narrow the ridges defining the wide trench cavities on which the micro lenses are formed. This means it difficult to further narrow the distance between the adjacent two of the micro lenses.
In Japanese laid-open patent publication No. 4-61277, there is disclosed a fourth conventional method of forming micro lenses by use of dies. FIGS. 4A through 4E are fragmentary cross sectional elevation views illustrative of solid-state image pick-up devices involved in a third conventional method of forming an alignment of micro lenses over photodiodes of the solid-state image pick-up device.
With reference to FIG. 4A, a die 132 is used which has cavities 131 which are aligned so that the adjacent cavities 131 are separated from each other by ridged portions 135. The shape of each of the cavities 131 defines the shape of the each micro lens to be formed. A transparent photo-curing or thermo-setting resin material 133 as a micro lens material is injected into the die 132.
With reference to FIG. 4B, a photo-curing or thermo-setting process is carried out to form a united micro lens group 136 which comprises a thin base layered portion 135 and an alignment of micro lenses 134 on the thin base layered portion 135.
With reference to FIG. 4C, the united micro lens group 136 is removed from the die 132.
With reference to FIG. 4D, the united micro lens group 136 is placed over a color filter 138 provided over a base structure 110.
With reference to FIG. 4E, the united micro lens group 136 is adhered onto the color filter 138.
The above fourth conventional method of forming the micro lens has the following problems. The united micro lens group 136 comprises a thin base layered portion 135 and an alignment of micro lenses 134 on the thin base layered portion 135. The thickness of the united micro lens group 136 corresponds to the total thickness of the thin base layered portion 135 and the micro lenses 134. The thickness of the united micro lens group 136 is thicker by the thickness xe2x80x9cZxe2x80x9d of the thin base layered portion 135 than the thickness of the individual micro lenses 134.
FIG. 5A is a view illustrative of an incidence of light through a camera lens into a solid-state image pick-up device having micro lenses formed in the fourth conventional methods illustrated in FIGS. 4A through 4E. FIG. 5B is a view illustrative of an incident of the light through micro lenses into photo-diodes of the solid-state image pick-up device in FIG. 5A.
With reference to FIGS. 5A and 5B, a solid-state image pick-up device is placed to be distanced from a camera lens 139. The solid-state image pick-up device has micro lenses 134. A light is transmitted through the camera lens 139 and further transmitted with spread toward the micro lenses 134. A center axis vertical to a plane of the base thin layered portion 135 of the micro lens group 136 penetrates a center of the camera lens 139. A micro lens 134b is positioned on the center axis. Micro lenses 134a and 134c are distanced from the center axis toward a direction included in the plane of the base thin layered portion 135 of the micro lens group 136. The light having been transmitted through the camera lens 139 is incident to the micro lens 134b at a vertical direction to the plane of the base thin layered portion 135 of the micro lens group 136. Namely, the micro lens 134b receives the vertical incidence of the light. The light having been transmitted through the camera lens 139 is incident to the micro lenses 134a and 134c at oblique directions to the plane of the base thin layered portion 135 of the micro lens group 136. Namely, the micro lenses 134a and 134c receive the oblique incidences of the lights. As described above, the thickness of the micro lens group 136 is thicker by the thickness xe2x80x9cZxe2x80x9d of the base thin layered portion 135 than the micro lenses 134. The thickness xe2x80x9cZxe2x80x9d of the base thin layered portion 135 increases the distance of the micro lens 134 from the photo receiving region 3. The micro lens 134b receives the vertical incident of light, whereby the light entirely reaches the photo receiving region 3. However, the micro lenses 134a and 134c receive the oblique incidents of lights, whereby the lights partially reaches the photo receiving region 3. As a result, the sensitivity is deteriorated.
The above fourth conventional method of forming the micro lenses has a further problem as follows. As illustrated in FIGS. 4A through 4E, the transparent photo-curing or thermo-setting resin material 133 as a micro lens material is injected into the die 132. Subsequently, the photo-curing or thermo-setting process is carried out to form the united micro lens group 136. The united micro lens group 136 is removed from the die 132, and then adhered onto the color filter 138 to form the solid-state image pick-up device. Since the united micro lens group 136 is prepared by the photo-curing or thermo-setting process, the united micro lens group 136 is hard, for which reason when the united micro lens group 136 is adhered onto the color filter 138, then it is possible that the color filter 138 receives a damage due to contact with the hard united micro lens group 136, whereby any wound may be formed on the color filter 138. The formed wound is projected on a screen. It may be possible to propose that in order to avoid the wound due to the contact of the color filter 138 with the hard united micro lens group 136, an outside wall is provided over the substrate for supporting the hard united micro lens group 136 which is floated from the color filter 138, so that an inter-space is formed between the hard united micro lens group 136 and the color filter 138. The inter-space is different in refractive index of light from the hard united micro lens group 136 and the color filter 138. This makes it difficult to design the shape and size of the micro lens and design the solid state image pick-up device. Further, the inter-space increases the distance between the micro lens and the photo-receiving region 3. This means decreasing the amount of the light, which have been transmitted through the micro lens 134a or 134c and reaches the photo-receiving region 3, whereby the sensitivity is further deteriorated.
In the above circumstances, it had been required to develop a novel method of micro lenses of a solid-state image pick-up device free from the above problem.
Accordingly, it is an object of the present invention to provide a novel method of micro lenses of a solid-state image pick-up device free from the above problems.
It is a further object of the present invention to provide a novel method of micro lenses of a solid-state image pick-up device, which improves an accuracy in shape and size of the micro lenses.
It is a still further object of the present invention to provide a novel method of micro lenses of a solid-state image pick-up device, which improves reliability of the solid-state image pick-up device.
It is yet a further object of the present invention to provide a novel method of micro lenses of a solid-state image pick-up devise, which improves sensitivity of the solid-state image pick-up device.
The present invention provides a method of forming micro lenses over a base structure of a solid state image pick-up device. The method comprises the steps of: forming a light-transmitting material layer on the base structure; and pushing a die having a die pattern against the light-transmitting material layer to transfer the die pattern of the die to the light-transmitting material layer, thereby forming micro lens patterns over the base structure.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.